We present direct numerical simulation results of a wave-current boundary layer in a current-dominated flow regime (wave driven to steady current ratio of 0.34) over bumpy walls for hydraulically smooth flow conditions (wave orbital excursion to roughness ratio of 10). The turbulent, wave-current channel flow has a friction Reynolds number of and a wave Reynolds number of. At the lower boundary, a bumpy wall is introduced with a direct forcing immersed boundary method, while the top wall has a free-slip boundary condition. Despite the hydraulically smooth nature of the wave-driven flow, the phase variations of the turbulent statistics for the bumpy wall case were found to vary substantially when compared with the flat wall case. Results show that the addition of weak waves to a steady current over flat walls has a negligible effect on the turbulence or bottom drag. However, the addition of weak waves to a steady current over bumpy walls has a significant effect through enhancement of the Reynolds stress (RS) accompanied by a drag coefficient increase of relative to the steady current case. This enhancement occurs just below the top of the roughness elements during the acceleration portion of the wave cycle: Turbulent kinetic energy (TKE) is subsequently transported above the roughness elements to a maximum height of roughly twice the turbulent Stokes length. We analyse the TKE and RS budgets to understand the mechanisms behind the alterations in the turbulence properties due to the bumpy wall. The results provide a mechanistic picture of the differences between bumpy and flat walls in wave-current turbulent boundary layers and illustrate the importance of bumpy features even in weakly energetic wave conditions.

Sliding force and punching pressure were contributing factors to widespread breakwater damage caused during the 2011 Great East Japan Tsunami (Takagi and Bricker, 2015), and were dominant factors causing displacement of caissons from the world’s deepest breakwater: the Kamaishi bay-mouth composite tsunami breakwater (Arikawa et al., 2012; Bricker et al., 2013). The current study focuses on understanding the physics necessary to correctly model the problem of breakwater over-topping by tsunami. To effectively model the physical behavior of the system, scaled model studies were carried out by Mudiyanselage (2017). The earlier numerical investigations carried out by Bricker et al. (2013) and Mudiyanselage (2017), did not prove conclusive to numerically model tsunami breakwater overflow using OpenFOAM employing a 2-D modeling approach. This was shown to be a major hurdle in prediction of the sliding force on the caisson due to the inability of modeling the non-aerated overflow jet over the caisson. Validation of the numerical model would allow parametric study of the flow physics for varying overflow conditions. As a result, a threefold approach of experimental model, analytical model, and numerical model studies was proposed. To achieve sufficient reliability and have complete flexibility, OpenFOAM was chosen for the numerical setup. This numerical model was used to validate the experiments carried out by Mudiyanselage (2017). The numerical model validates and reproduces the flow physics very well. Overall, the numerical results indicate that non-aeration could provide about 8-19% additional force. It was observed that the force on the caisson has a periodic fluctuating behavior. Additionally, the aeration mechanism and overflow jet breakup during the flow was also investigated. It was observed that the highly 3-dimensional behavior of the overflow jet results in the aeration of the cavity underneath the jet. This also explains why the previous studies Bricker et al. (2013) and Mudiyanselage (2017) failed to correctly model the overflow jet using a 2-D modeling approach.